Fix NS crash on invalid frame title string (bug#42904)

Fix NS crash on invalid frame title string (bug#42904)

Instead of blindly assuming that all Emacs strings are valid UTF-8,
which they are not, use a more careful conversion going via UTF-16
which is what NSString uses internally.  Unpaired surrogates will
still go through to the NSString objects, but the NS libs handle them
gracefully.

* src/nsterm.h (EmacsString): New category.
* src/nsfns.m (all_nonzero_ascii): New helper function.
([NSString stringWithLispString:]): New method.
(ns_set_name_internal): Use new conversion method.